The subject invention relates to ink jet printing. (As used herein, the term xe2x80x9cink jet xe2x80x9cSprintingxe2x80x9d refers to any form of printing wherein print control signals control a print mechanism to eject ink drops to produce a matrix of pixels, i.e. picture elements, having two or more intensity values to represent an image.) More particularly it relates to apparatus and methods for monitoring the operation of an ink jet printhead.
Low cost, widely available ink jet printing technologies such as bubble jet, and piezoelectric ink jet printing have enabled many new applications where dynamically varying information must be transmitted in printed form. Many of these applications rely upon a consistent level of print quality over time since the failure to capture the unique information on even a single document can have serious consequences.
A particular example of an application of ink jet printing where a consistent level of print quality is very important is the use of digital print mechanisms in postage meters and mailing machines. As is well known such devices print postal indicia on mailpieces as proof of the payment of postage. Upon payment to a proper authority such meters or machines are xe2x80x9cchargedxe2x80x9d with a representation of an equivalent amount of funds. As postal indicia are printed the funds in the meter are debited accordingly until exhausted. Since postal services accept indicia printed by postage meters or mailing machines as conclusive proof of payment of the amount of postage indicated such devices are in effect machines for printing money. As a result postal services have imposed high standards both on the print quality of indicia produced by such machines, and on the design of the machines themselves to assure that the appropriate amount is debited from the amount charged into the machine for each indicia printed.
Low cost ink jet printing technologies have greatly simplified and improved the design of postage meters and mailing machines in many respects. Prior postage meters and mailing machines relied upon impact printing techniques which required complicated and expensive mechanisms to print varying postage amounts, which can now be printed in a simple, conventional manner with ink jet print mechanisms. More importantly, ink jet print mechanisms can be easily programmed to print other information such as security codes or addressing or tracking information with the postal indicia to facilitate automated mail handling. However such low cost ink jet print mechanisms can not easily provide consistent print quality as their mechanisms tend to degrade over time as ink dries up, small print nozzles clog or one or more of a number of small, rapidly cycling print elements fails. Such failure can cause substantial losses to a mailer since a large number of mail pieces of substandard print quality may be rejected by a postal service after the cost of the postage has been debited from the prepaid amount charged to the machine.
U.S. Pat. No. 4,907,013; to: Hubbard et al.; issued: Mar. 6, 1990 is believed to be the prior art closest to the subject invention and relates to circuitry for detecting failure of one or more nozzles in an ink jet printhead. In Hubbard et al. a line containing one dot printed by each nozzle in the printhead is scanned to detect the possible absence of a dot. The line can form either a test pattern run before the start of a printing operation or can be incorporated into the image to be printed.
U.S. Pat. No. 5,038,208; to; Ichikawa et al.; issued: Aug. 6, 1991 teaches an ink jet printer which stores the image forming characteristics of an ink jet printhead and which corrects the image forming signals in accordance with the stored characteristics to maintain uniform print density.
U.S. Pat. No. 5,126,691; to: Millet et al.; issued: Jul. 7, 1992 is similar to Hubbard et al. in that it teaches a method for monitoring print quality by the use of a specially printed control frame.
U.S. Pat. No. 5,321,436; to: Herbert; issued Jun. 14, 1994 teaches a postage meter in which the operation of an ink jet printhead is checked by printing a predetermined bar code and then scanning the bar code to determine if it was correctly printed.
U.S. Pat. No. 5,473,351; to: Heterline et al. teaches a method and apparatus for monitoring print density by measuring printed line width and modifying the energy of the pulses applied to each ink jet nozzle to correct the line width.
Commonly assigned U.S. patent application, Ser. No. 09/046,902; titled: Mailing Machine Including the Prevention of Loss of Funds; filed Mar. 24th 1998, which is hereby incorporated by reference, teaches a postage meter or mailing machine having a capability for generating a test pattern; where the test pattern includes pseudo-random information unknown to an operator. Failure of the operator to correctly input the information causes the postage meter to be disabled; and correct input of the information enables the postage meter to continue operation.
While perhaps suitable for their intended purpose the print quality monitoring and control techniques found in the prior art did not provide a simple and inexpensive way to directly monitor operation of ink jet printheads. In general the prior art require expensive apparatus for sensing and measuring specially selected print patterns, together with complicated control of the printhead drive signals.
Thus it is an object of the invention to provide an improved apparatus and method for monitoring operation of an ink jet printhead so that prompt corrective actions can be taken.
The above object is achieved and the disadvantages of the prior art are overcome in accordance with the subject invention by means of an apparatus and method for monitoring operation of an ink jet printing mechanism which include providing predetermined control signals to the ink jet printing mechanism, the printing mechanism responding to the control signals to eject a curtain of ink drops through a predetermined gap; a source of illumination projecting a beam through the gap at substantially a right angle to the path of the curtain, whereby the intensity of the beam downstream from the gap is reduced proportionally to the curtain""s optical density. The beam is sensed downstream from the gap to generate an optical density signal representative of variation in intensity of the beam, whereby the optical density signal is representative of the optical density of the curtain; the optical density signal is compared to a first reference signal; and, if the comparison indicates that the curtain is insufficiently dense, a printhead malfunction signal is generated.
In accordance with an aspect of the subject invention, the printing mechanism is comprised in a postage metering system and is further controlled to print postal indicia.
In accordance with another aspect of the subject invention, the postage metering system is responsive to the printhead malfunction signal to inhibit further printing of postal indicia.
In accordance with another aspect of the subject invention, the curtain is oriented so that its long axis is at approximately a right angle to the beam.
In accordance with another aspect of the subject invention, the beam is optically altered to form a collimated beam in the gap and focused onto an aperture of a photosensor to generate the signal.
In accordance with another aspect of the subject invention, the gap is substantially surrounded with a transparent dust ring having openings for entry and exit of the curtain, the beam passing through the dust ring as it enters and exits the gap.
In accordance with a related aspect of the subject invention, the surface of the dust ring is shaped to carry out the optical altering and focusing steps.
In accordance with another aspect of the subject invention, the curtain is oriented with respect to the beam so that a foreshortened projection of the curtain substantially coincides with an aperture of the photosensor.
In accordance with another aspect of the subject invention, the beam is masked so that the masked beam substantially coincides with an aperture of the photosensor.
In accordance with another aspect of the subject invention, the curtain is oriented so that its long axis is approximately parallel to the beam.
In accordance with another aspect of the subject invention, during a calibration period, in which no curtains pass through the gap, the beam""s intensity is electronically varied to artificially generate the optical density signal; and the optical density signal is fed back to the illumination source to control the beam""s nominal intensity; so that the nominal intensity is varied to compensate for variations in overall sensitivity.
In accordance with a related aspect of the subject invention, during measuring periods in which the curtains pass through the gap, the nominal intensity is fixed.
Other objects and advantages of the subject invention will be apparent to those skilled in the art from consideration of the detailed description set forth below and the attached drawings.